a. Field of the Invention
This invention relates to laser-illuminated active viewing systems.
b. Description of Related Art
Passive night vision imaging devices, based on microchannel plate (MCP) image intensifiers, are capable of up to a million fold multiplication of available light. The phosphor screen output of such devices can be directly coupled or imaged onto a video camera for viewing on a TV monitor. However, passive night vision devices have severe limitations. Frequently the amount of ambient light is insufficient for proper target illumination, and targets cannot be identified, especially at long ranges. In that event, active illumination is required.
Active illumination systems based on gated image intensified cameras coupled with pulsed lasers have existed for several years. The gating of the intensifier inside the camera allows the foreground and/or background of a target to be removed from the field of view of the camera. The benefit of foreground removal is that the atmospheric backscatter from the pulsed laser source is eliminated yielding a much greater signal to noise ratio (S/N) of the target of interest and the ability to silhouette the target.
The camera gate is controlled by a voltage applied to the camera intensifier. 250 volts is required to shutter a Gen II image intensifier tube which provides a minimum gating time of about 5 nanoseconds due to the large voltage, parasitic capacitance, and the impedance of the voltage source. The best response for the photocathode for these cameras is in the 350-900 nm range with sensitivity peaking in the 500 nm spectral range. A Gen III image intensifier tube can also be considered because it has a higher quantum efficiency, but due to its construction, the gating rise and fall times are significantly longer for the identical driving impedance. A Gen III device requires switching about 700 volts which increases the minimum gating time to about 70 nanoseconds for conventional devices. Special devices, however, can be constructed with faster response times. The photocathode response for these tubes is from 600 nm to 900 nm with a peak sensitivity at about 820 nm. The QE of current Gen II devices is in the range of 8% while the Gen III devices can be as high as 30%. A typical laser used for this application is a pulsed laser diode or an alexandrite laser for imaging at much greater distances.
It is also possible to use a new class of Transferred Electron (TE) photocathodes which have responses out to 1.7 .mu.m. These devices can be constructed with internal microchannel plate intensifiers or connected to a device to produce a rapid gateable intensified CCD array camera. Laser sources for these include tunable Optical Parametric Oscillators (OPO's) which can cover the entire spectral region.
U.S. Pat. No. 4,642,452 to Loy discloses an integrated active night vision system consisting of a low power near infrared illumination source, an intensified receiver and monitor. Illumination is provided by low power light flashes of very short duration. The light source is either a diode laser which emits pulses on the order of a microsecond in the near infrared, or a flashlamp with output pulses of 10-20 milliseconds duration which are passed through an infrared filter. The intensifier is gated to reduce atmospheric backscatter. The system disclosed, however, suffers from poor range resolution.
The gaussian temporal shape of the outgoing laser pulse of existing active imaging systems can give some range information, but it is limited. Accordingly, it would be advantageous to obtain more accurate distance and depth information about a target being viewed from large distances. This includes information about the size of the target, and separation of features located on the target, as well as distance of the target from the camera/laser arrangement.